Glass fibre products, plastics and synthetic materials used outdoors need to be protected against the sun's damaging ultraviolet rays and temperature cycling in the presence of moisture that lead to yellowing and fading of surfaces. For instance is it desired that sailboats, motorboats, yachts and mobile homes keep looking their best in all weathers, withstanding the effects of sun, salt water and rough handling. To preserve the finish, gelcoats are usually applied to exterior surfaces and parts where weathering resistance is needed. Aside from having the right hardness, mechanical strength, flexibility and stability under fluctuating temperatures, the gelcoat must also provide some weathering resistance.
Gelcoat compositions that are to be exposed to UV light and rain for a long period of time are required to be superior in weatherability. Such exposure causes the binder resin of the gelcoat to be degraded so that the gelcoat becomes dull in gloss and discolored in a short term. In addition, the gelcoat undergoes chalking and is degraded or cracked with the passage of a long period of time.
Polyester gelcoats usually comprises a polyester on which the gelcoat is based and various additives. The use of unsaturated polyester resins as a base in gelcoats for outdoor glass fibre surfaces is known from e.g. U.S. Pat. No. 6,617,417 and U.S. Pat. No. 6,583,218. Suitable polyester resins for gelcoats include linear polyester resins, branched polyester resins, copolymeric polyester resins and mixtures or blends thereof. The utility of such compositions in outdoor applications is often limited by the resistance of the resin composition to weathering.
Unsaturated polyester resins are generally crosslinked with a compatible crosslinking monomer such as styrene, vinyl toluene, methyl methacrylate, methyl styrene, divinyl benzene, diallyl phthalate and the like. Usually the amount of crosslinking monomer is about 10 percent to about 55 percent by weight of the unsaturated polyester resin. Flexible unsaturated polyesters for coating applications have been traditionally processed from dicarboxylic acid and polypropylene oxides or polyethylene oxides. Part of the acid has to be unsaturated in order to have curing properties with normal peroxides or other radical initiators. Typical improvements relate to variations of monomers and by copolymers, e.g. blending unsaturated polyester with vinyl esters and epoxy resins.
Examples from prior art uses of unsaturated polyester resins in gelcoats include U.S. Pat. No. 6,617,417 which concerns an unsaturated polyester resin composition comprising divinyl ethers which are curable at room temperature. This composition comprises less than 30 wt. % of styrene, and do not provide improved yellowing resistance. U.S. Pat. No. 5,741,448 concerns a shrink resistant resin composition, which is curable at room temperature. The composition comprises a curable unsaturated polyester resin, an accelerator, a low temperature free radical peroxide initiator and a low profile additive comprising polyolefin powder. The composition is not suitable as a weathering resistant gelcoat. U.S. Pat. No. 6,583,218 concerns an unsaturated polyester resin composition with a reduced monomer content. The gelcoat is suitable for marine coating applications. Reduced styrene content reduces the release of volatile unsaturated organic monomers to the atmosphere but does not provide resistance to yellowing.
Gelcoats with good weatherability and resistance to yellowing are also known from prior art. U.S. Pat. No. 7,078,476 discloses a coating comprising a copolyester resin having superior gloss retention, weatherability and colour stability after long term outdoors exposure. The copolyester is not unsaturated, does not comprise styrene, and has an acid value of 0-3 mg KOH/g and is mainly suited as binder for an outdoor-durable top-coating paint. U.S. Pat. No. 5,464,909 concerns the use of an aliphatic polyester providing good UV resistance and weatherability. The polyester resin is based on dimethyl 2,6-decalindicarboxylate and neopentyl glycol and is used in powder coatings suitable for adhesion to metal substrates. EP 0 372 264 concerns one-component urethane compositions with improved weathering properties. The coating compositions comprise at least twenty percent by weight of resin solids of a compound comprising at least one oxazolidine ring chemically combined from the nitrogen atom of the ring through an alkylene-urethane linking group to a tetramethylxylylenediisocyanate prepolymer radical. EP 1 203 783 concerns the use of unsaturated polyester resins in products showing improved fire retardancy. Good weathering performance is obtained due to the use of unsaturated polyesters rather than phenolics. The products are not suited for gelcoats on glass fibre. EP 0 787 769 concerns modified polycarbonate/polyester compositions with high weather resistance. The compound comprises an aromatic polycarbonate resin, and a polyolefin rubber graft copolymer and a core-shell copolymer.
For gelcoats comprising an unsaturated polyester resin, it is common to use additives such as UV-stabilisers, thixotropic agents, thixotropy enhancers, suppressants, surface tension agents, co-promoters, air release agents, filler, wetting agents, levelling agents and pigments. Some of these additives are added to provide UV resistance and colour stability and hence increase the weatherability of the final coat. Traditionally, UV stabilisers are added to the gelcoat to provide the desired resistance to yellowing. However, the effect of added agents providing increased weatherability is limited.
Therefore, it is desirable to base the gelcoat on a polyester that in itself possesses good weatherability. The present invention presents the use of a polyester gelcoat specially formulated for improved resistance to UV and yellowing and retention of gloss.